For Mitt Romney, the Wisconsin primary has looked at times like a buddy movie with Paul Ryan.

The front-runner and House Republican congressman have been together non-stop. They've attended fish fries, interrupted each other at town halls, even gagged each other on April Fools Day (Romney on the receiving end).

At the root of it all, a shared emphasis on conservative values, health care reform and tax plans has permeated every appearance. When asked to describe his plans for changing the tax code on Monday, Romney handed the mic to Ryan, saying "I heard the congressman answer this question better than I can last time we chatted. So I'm going to have him describe, just for a moment, his plans on the tax code, which are very similar to my own."

As Romney rakes in delegates from Wisconsin, Maryland and Washington, D.C.'s primaries on Tuesday, and party ranks close in behind him as he approaches the magic number of delegates needed to win the nomination, the focus will turn to the general election and Romney's first big test: Who he will pick as his running mate?

There will be talk of who comes from a swing state, who has the right kind experience and who has the right ideological credentials -- but an important and overlooked factor may be the personal connection of the kind that was on display with Ryan.

Romney, who has had trouble connecting with voters on the campaign trail, is simply a better candidate when he's got a sidekick. The best example of this is his wife. Whenever she introduces him, he is beaming and confident, giddily watching as the crowd turns captive to her.

With others though as well, he also comes alive in a way that contrasts with his usual stiff performance. While his own stump speech hardly deviates, he enjoys having a rotating cavalry of prominent Republicans with him at events, such as New Jersey Gov. Chris Christie or Arizona Sen. John McCain, who each bring a brashness and humor to the stump that Romney himself knows he does not.

Still, he is looser, trying his own jokes and playing off his second. "What you hope for is someone to heckle him," Romney said of Christie, before a Cleveland crowd in March. "He'll walk up to the stage like this," he said, sauntering on stage and wagging his finger like the governor. "He looks at them and tells them what to."

Chumminess won't be the only criteria, of course. Romney has given some hints about how he'll make his selection. Perhaps learning from one of the greatest criticisms of McCain's choice of Sarah Palin in 2008, Romney often says that if he wins the nomination, his choice for vice president must be someone who has the experience to lead and likeminded pursuits as him should he or she ever be in that position. That would seem to count out Marco Rubio, perhaps the most talked about potential running mate but who, at age 41, doesn't have much experience and little of it in positions of executive leadership. This is also a potential problem for Rep. Ryan.

South Dakota Sen. John Thune and Ohio Sen. Rob Portman, both of whom have done a good deal of campaigning for Romney, meet that criteria and are well liked in the heartland -- sure to be a battleground in the general. Gov. Bob McDonnell of Virginia has only been in office a little more than two years, but he has a broad military background that Romney proud touts on the trail.

And despite the fact that Indiana's Gov. Mitch Daniels has said he is not endorsing anyone in the primary process, Romney told Indianans in the fall that Daniels could be "wherever he'd like to be," in his cabinet -- adding that "Mitch is an extraordinarily capable guy."

Former New Hampshire Senator John E. Sununu, who has not endorsed Romney, had an op-ed in Tuesday's Boston Globe encouraging the front-runner to resist choosing someone flashy and exciting to goose his poll numbers, favoring instead an established politician already well-known -- and well-vetted. "Let us embrace the obvious: The winning choice is the dull choice -- a running mate the public already knows, warts and all," Sununu wrote.

In an interview Tuesday morning on MSNBC, Sununu's father, John H. Sununu, the former New Hampshire governor and chief of staff to President George H. W. Bush, was asked about the piece, which argued against a risky pick like former Vice President Dan Quayle.

"I think he's learned a lot from his father," the elder Sununu grinned.